In the kraft pulping industry in which cellulose containing raw material is treated at high temperatures with cooking chemicals to yield cellulose pulp, it is of vital economic and environmental importance to recover and regenerate the spent cooking liquor with its dissolved wood substance as well as the cooking chemicals. This is achieved by extracting spent (black) cooking liquor from the digesters and further by washing the pulp discharged from the digesters with water, evaporating the liquor obtained and then combusting the evaporated liquor in a recovery boiler. From the bottom of the recovery boiler a smelt is taken out and dissolved in water to form a soda liquor (green liquor) which is a solution of mostly sodium carbonate and sodium sulphide. Alternatively, the soda liquor may be produced by substoichiometric gasification of the spent cooking liquor. The sodium carbonate content of the soda liquor is converted to sodium hydroxide by the addition of burned lime (CaO) in the so called causticizing process. In the causticizing process the burned lime forms insoluble lime mud (CaCO3) which is separated from the caustic liquor—called white liquor—in a subsequent filtration step. The white liquor is thereafter reused as cooking liquor in the kraft cooking of the cellulose containing raw material.
The chemical reaction in the causticizing process proceeds in two reaction steps. In the first reaction step, usually carried out in an atmospheric so called lime slaker, the burned lime consisting mostly of calcium oxide is slaked by the water content of the green liquor to form hydrated lime.CaO(s)+H2O->Ca(OH)2(s)  (1)
This first reaction is called the slaking reaction and it is rapid and strongly exothermic.
In the second reaction step the dissolved carbonate in the green liquor combines, conventionally in several atmospheric agitated reactor tanks in series, with the calcium in the slaked lime to form lime mud. At the same time the hydroxide content of the slaked lime goes into solution.Ca(OH)2(s)+CO32−⇄CaCO3(s)+2OH−  (2)
This second reaction is slower than the first reaction and is also an equilibrium reaction which means that all the dissolved carbonate will not react even if a surplus of burned lime is added.
It is further known in the art, from for example SE 504 212, to perform the causticizing at elevated pressure and temperature. The elevated temperature makes the causticizing reaction (2) proceed faster and the pressurised design prevents boiling and thus loss of the heat of reaction of the exothermic slaking reaction. Thus the heat of reaction can be preserved at the same time as it is utilised to increase the reaction rate of the second, slower reaction.
From U.S. Pat. No. 4,627,888 it is known to perform a pressurised slaking process where the soda liquor to be causticized is divided into two parts, the first part being used for the slaking and the second part being added after the slaking for the actual causticizing process.
It has also been shown that if the product white liquor/lime mud slurry is kept under elevated pressure and temperature during a succeeding separation step, a further benefit can be elicited. As the viscosity of the white liquor is lower at higher temperature the capacity of a given filter can be higher at this temperature resulting in reduced filter size for a given capacity.
In conventional recausticizing systems, whether atmospheric or pressurised, the storage silo for burned lime is placed directly above or adjacent to the lime slaker and the burned lime is being fed to the slaker by one or several solids materials conveyors. This solution has a some disadvantages. If placed directly above the slaker, the structure for the storage silo becomes expensive as will the conveying system of burned lime from the lime kiln (where lime mud is reburned to CaO) to the silo. If placed more remotely from the slaker and in close vicinity to the lime kiln discharge, the conveyors to the slaker will instead become expensive. These disadvantages could be overcome if the lime slaker reactor could be fed by pumping burned lime from the storage silo as a slurry in green or white liquor. However such pumping of slurry does in itself have its difficulties:                The lime slaking reaction is rapid and strongly exothermal so if slurried in hot liquor the liquor may be brought close to or to boiling reducing the available net pressure suction height (NPSH) for the slurry pump. This may cause the slurry pump to cavitate.        The burned lime often contains oversize material, refractory or metal trash that may block or damage the feed pump or piping.        
Other problems that are identified in conventional recausticizing systems are e.g. that feed-back control is complicated by the slow reaction (2) which is performed in a series of vessels and that the process equipment requires a large space.
The present invention provides a feeding device in a slurry pumping system that overcomes the above difficulties, and also provides a process which utilizes said feeding device.
Hence, there is provided a feeding device for feeding burned lime to a reaction vessel for causticizing a soda liquor to caustic soda, wherein said feeding device, in its upper part, comprises an inlet for a slurry of said burned lime and said soda liquor, or inlets for said burned lime and said soda liquor, respectively, for enabling formation of said slurry inside said feeding device, and wherein said feeding device comprises a feed vessel of tall and slender shape, which feed vessel in its lower part comprises an outlet for said slurry, which outlet, via one or more pump(s), is operatively connected to said reaction vessel.
The feed vessel preferably presents an internal liquor level of at least 1.5 meters, preferably at least 2 meters and most preferably at least 3 meters, whereas its inner diameter depends on the size of the slurry flow through the feed vessel and on the retention time, which is 10–150 seconds, preferably 20–120 seconds and even more preferred 30–60 seconds, giving an inner diameter of about 0.1–1.5 meters, preferably 0.2–1 meters and most preferably 0.3–0.8 meters. By the tall and slender shape of the feed vessel, a shape that is commonly referred to as a stand pipe, there is created a possibility to form a pumpable slurry of the burned lime and soda liquor, which slurry thus is formed in a volume which is small enough to prevent the exothermic reaction (1) from proceeding far enough to cause boiling, at the same time as a hydrostatic pressure, due to the height of the liquor level in the feed vessel, prevents the downstream one or more pump(s) from cavitating.
According to one aspect of the invention, the feeding device, in said upper part thereof, comprises a cyclone including said inlets for the burned lime and the soda liquor, respectively, for enabling formation of said slurry inside the cyclone or just below the cyclone.
According to another aspect of the invention, the feeding device further comprises a trash trap, which trash trap preferably constitutes a liquid filled branch-off to said feed vessel, which trash trap comprises means for discharging coarse burned lime or other non desired solid material, and which trash trap also comprises an inlet, at the bottom part thereof, for soda liquor and/or comprises an agitator device, such as for example a propeller or other agitator paddle.
According to yet another aspect of the invention, the one or more pump(s) is/are especially designed to handle a slurry with a substantial amount of coarse burned lime and at least one of the pumps (preferably the first one) is designed to crush oversized burned lime between a rotor part and a stationary part in the pump.
The invention further presents a process for reacting a soda liquor with burned lime, by combined slaking and causticizing, for production of caustic soda, comprising the steps of                (a) forming a slurry of said burned lime and a first part of said soda liquor and allowing said slurry a retention time of 10–150 seconds in a feed vessel,        (b) preheating a second part of said soda liquor,        (c) combining said slurry from step (a) and said preheated second part of said soda liquor of step (b),        (d) maintaining the combined slurry and liquor at an elevated pressure and at an elevated temperature for completion of the reaction between the burned lime and the soda liquor to yield caustic soda and lime mud.        
According to one aspect of the process of the invention, said first part of said soda liquor constitutes about ⅓ to ½ of the entire amount of soda liquor which participates in the reaction, whereas said second part of said soda liquor constitutes about ½ to ⅔ of the entire amount of soda liquor which participates in the reaction. By dividing the soda liquor into these two streams, there can be created a slurry of the burned lime and the first part of the soda liquor, which slurry, due to the short retention time of step (a), will not have time to completely undergo the exothermic reaction (1) above before it is pumped into the pressurised reaction vessel, where the reactions (1), (2) are completed. By preheating a second part of the soda liquor, preferably by indirect heat exchange against a product caustic soda (white liquor), the reaction rate is further speeded up so that the reactions can be completed in a very short period of time and in process equipment which is much smaller and less space requiring than in conventional systems.
According to another aspect of the invention, said feed vessel is filled with enough liquid/slurry to provide a hydrostatic pressure high enough prevent boiling due to an exothermic reaction between said burned lime and said soda liquor.
According to yet another aspect of the invention, the process proceeds with the steps of,                (e) allowing coarse, unreacted, burned lime to settle and thereafter discharging it,        (f) filtering said lime mud and caustic soda under elevated pressure and elevated temperature, preferably about the same temperature as in step (d), in order to separate said caustic soda from said lime mud.        
The invention further presents a feeding system for feeding burned lime to a reaction vessel for causticizing a soda liquor to caustic soda, which feeding system comprises:                (aa) a feeding device, which feeding device includes an inlet for a slurry of said burned lime and a first part of said soda liquor, or inlets for said burned lime and said first part of said soda liquor, respectively, for enabling formation of said slurry inside said feeding device,        (bb) one or more pump(s), which are arranged to pump the slurry from the feeding device in (aa) to said reaction vessel,        (cc) a heater, which is arranged to heat a second part of said soda liquor,        (dd) a distributing device, which is arranged to distribute said first part of said soda liquor to the feeding device in (aa) and to distribute said second part of said soda liquor to the heater in (cc), and        (ee) a combining device, which is arranged to combine said slurry, before or in connection with its inlet into said reaction vessel, with said heated second part of said soda liquor.        
The advantages of the feeding device and the process described above are several:
The pump-in feed system allows the burned lime storage silo to be placed lower than in a conventional system where the lime is added by gravity to the first of several reactor tanks.
The pump-in feed system utilising the feeding device allows the burned lime storage silo to be placed away from the reactor tank also facilitating retrofits of this system in existing plants.
The very high reaction rate of this process compared to the conventional makes the required reactor volume much smaller and thus gives lower space requirement and also lower equipment cost.
The high reaction rate makes feed-back control of the lime dosage more accurate as the time lag from dosing to completed reaction is greatly reduced.
The small reactor dimensions and fully pressurised reactor design greatly reduces the heat losses and thus makes control of the lime dosing based on measurement of the temperature rise caused by the exothermic slaking reaction (1) an accurate and simple control mechanism.
The high temperature during the white liquor/lime mud separation makes the filter area required smaller and thus reduces the size and cost of this filter.